This invention relates to the joining of dense bodies of refractory metal such as tungsten or molybdenum to carbonaceous bodies, and more particularly to the employment of reaction brazing at high temperature to join dense bodies of tungsten or molybdenum or alloys thereof to carbonaceous supports, such as graphite or carbon-carbon composites. Even more particularly, the invention relates to the reaction brazing of x-ray generating anodes made primarily of either molybdenum or tungsten, to graphite supports or to carbon matrix, carbon fiber-reinforced composite supports to produce assemblies suitable for use at high temperature in a vacuum environment where temperature cycling will be experienced and collectively would tend to result in undesirable chemical reactions, e.g. carbon diffusion from the support and formation of substantial metal carbide.
There are various applications where it is desirable to attach a tungsten or molybdenum refractory body to a carbonaceous substrate in a manner so as to create an effective joinder that will remain satisfactory for extended periods in a high temperature environment, i.e. above 1000xc2x0 C. and in certain instances above 1500xc2x0 C. or even above 2000xc2x0 C. One such use of such structures is in the field of rotating x-ray anodes, and U.S. Pat. No. 3,579,022 shows the creation of a rotary anode for an x-ray tube wherein a tungsten-rhenium alloy is bonded to a graphite base by first depositing a thin stratum of rhenium having a thickness of a few micrometers. In U.S. Pat. No. 3,649,355, a graphite base for a rotary x-ray anode is first plasma-sprayed with tungsten to produce a coating of substantially pure tungsten, or an alloy thereof with rhenium, osmium or the like. Thereafter, an outer layer of tungsten is preferably deposited from a gaseous phase using CVD or the like. In U.S. Pat. No. 3,689,795, a molybdenum or molybdenum alloy base having a thickness of about 6 millimeters is used, and a focal track of pure tungsten or a tungsten-rhenium alloy is applied thereto by chemical vapor deposition (CVD) or a like process. To improve the crack resistance of such a molybdenum base, it is suggested to form the base using powder metallurgical techniques from a mixture of metal powders so the base will contain from 50 to 500 ppm of boron.
U.S. Pat. No. 4,132,917 shows a graphite body which has a metal band brazed thereon for a focal track. Illustrated in the patent is the use of a molybdenum or molybdenum alloy layer which is contiguous with the graphite body, and a layer of a tungsten-rhenium alloy that is superimposed thereon. In one embodiment, a thin coating of titanium carbide is applied to the graphite by CVD before brazing a metallic ring of the desired shape using Ti or Zr foil or powder paste, which ring may be formed by a powder metallurgical process.
U.S. Pat. No. 4,516,255 discloses the use of a rotating x-ray anode made from a molybdenum alloy containing some carbon, such as TZM, which is provided with a focal path of tungsten or a tungsten alloy. Using plasma spraying or the like, an oxide coating, such as titanium oxide, is formed on the TZM body, preferably after an intermediate layer of molybdenum (Mo) or tungsten (W) having a thickness between 10 and 200 xcexcm has been applied by plasma spraying or the like.
U.S. Pat. No. 4,990,402 teaches joining a metal part to a fiber-reinforced pyrolytic graphite structure or the like such as a structure wherein which the fibers are irregularly arrayed. In order to solder a molybdenum alloy component, such as TZM, thereto, a solder is used which is 70% silver, 27% copper and about 3% titanium.
Although such methods of joinder have proved reasonably effective for certain applications, the search has continued for improved methods of bonding dense refractory metal bodies, such as those of tungsten, molybdenum and their alloys, to carbonaceous supports by creating bonds that will exhibit excellent high temperature stability over a long term even in the face of a relatively substantial difference in coefficients of thermal expansion that would tend to create stresses at such a joint, while also resisting diffusion of carbon from such support into the dense refractory metal body. In addition, when the joinder is of an x-ray anode to a support, the thermal conductivity through the joint should preferably be adequate so that it does not create a heat flow choke that would deter heat being generated from flowing freely away from the anode, and the heat capacity of the support and its emissivity should be adequate to dissipate heat transferred to it.
The invention provides methods for joining bodies of refractory metal in elemental form to carbonaceous supports in a manner that creates a bond which is capable of withstanding temperatures at least as high as 1300xc2x0 C. and preferably of at least 1500 to 1600xc2x0 C. for substantial lengths of time, even higher temperatures for short periods, and perhaps more importantly of being able to withstand frequent cycling between far lower temperatures, e.g. close to room temperature, and such high temperatures. Alternative methods to certain preferred methods produce bonds which are capable of operating at a temperature of about 2000xc2x0 C. or above.
In certain of these preferred methods, a Reactive metal, preferably in the form of a foil, and a powder mixture containing a boride of the refractory metal being joined, a carbide of the Reactive metal, and preferably additional elemental metal, e.g. of the foil and/or the metal body, are introduced between complementary surfaces of the bodies being joined. This assembly is then heated to a reaction-brazing temperature (as hereinafter more specifically defined). For joining dense tungsten bodies, e.g. those made of single crystal tungsten or the like, an alcohol slurry of particles of tungsten boride plus a carbide of a Reactive metal, e.g. hafnium (Hf), carbide and/or zirconium (Zr) carbide, may be applied to the carbonaceous support, which slurry may also include some of these metals in elemental form. When a Mo or Mo alloy body is being joined, Mo boride is substituted for W boride. Reactive metal in the form of a paste or preferably a foil is juxtaposed with the W or Mo surface to be joined. The slurry does preferably contain powder of the Reactive metal of the foil (and also the metal of the carbide should it be different), and it also preferably contains W or Mo powder (depending upon the body being joined). The method produces a strong joint of low thickness having good thermal conductivity that is particularly valuable in the construction of an x-ray anode. The carbonaceous substrates may, for example, be dense graphite bodies or carbon-carbon composites wherein either bundles of carbon fibers or carbon filaments from woven cloth are oriented in a direction transverse to the surface of the dense refractory body being joined, which composites may also contain fibers oriented parallel to such surface.
In a particular aspect, the invention provides a method of making an x-ray tube target anode, which method comprises providing a dense body of tungsten (W) or molybdenum (Mo) metal suitable for serving as a target anode to create x-rays, providing a carbonaceous support body capable of withstanding high temperatures under vacuum conditions and having a surface complementary to a surface of said dense body, coating said complementary surface of said support body with a layer of a material containing a mixture of particulate Hf carbide or Zr carbide and particulate tungsten boride or molybdenum boride, and joining said dense body to said carbonaceous anode support by introducing a layer of elemental hafnium (Hf) or zirconium (Zr) between said complementary surfaces of said support body and said dense body, juxtaposing said complementary surfaces of said dense body and said support, and heating to a reaction-brazing temperature under vacuum or an inert atmosphere such that said dense body thereafter strongly adheres to said carbonaceous support body. The resultant product allows good heat flow from the anode body into the support at its high temperature of operation.
In a more particular aspect, the invention provides a method of joining a dense tungsten(w) or molybdenum(Mo) body to a carbonaceous support, which method comprises providing a dense W or Mo metal body, which body has one surface designated for joinder to another body, providing a carbonaceous support body capable of withstanding high temperatures in the absence of air, which support has a surface complementary to said designated surface of said dense body, coating said complementary surface of said support body with a layer of a material comprising a mixture of particles of a refractory metal boride and of a metal carbide, providing a Reactive metal layer and juxtaposing said two complementary surfaces with said layer therebetween, and joining said dense body to said carbonaceous support body by heating to a reaction-brazing temperature such that said refractory metal body thereafter strongly adheres to said carbonaceous support while an intermediate barrier forms between said two bodies which thereafter diminishes diffusion of carbon from said support body into said refractory metal body.
In a still more particular aspect, the invention provides a method of joining a dense tungsten(w) or molybdenum(Mo) refractory metal body to a carbonaceous support, which method comprises the steps of providing a dense W or Mo refractory metal body, which body has one surface designated for joinder to another body, providing a carbonaceous support capable of withstanding high temperatures in the absence of air, which support has a surface complementary to said designated surface of said dense body, coating said complementary surface of said support with a layer of a material comprising a mixture of particles of a boride of the refractory metal of the body and of a Reactive or refractory metal carbide, juxtaposing said two complementary surfaces, and joining said refractory metal body to said carbonaceous support by heating to a reaction-brazing temperature of at least about 2200xc2x0 C. such that said refractory metal body thereafter strongly adheres to said carbonaceous support and an intermediate barrier forms therebetween which diminishes diffusion of carbon from said support into said refractory metal body both during said joining step and later during use of said body in a high temperature environment.